Process for treating metal surfaces



Oct. 27, 1964 Filed March 8, 1962 Cafe United States Patent O 3,154,438 PROCESS FR TREATENG METAL SURFACES Heinz Keller, Friedrich Kaysser, Werner Rausch, and

Walter Stenger, Frankfurt am Main, Germmy, assignors, by mesne assignments, to Hooker Chemical Corporation, New `York, NKY., a corporation of New York Filed Mar. 8, 1962, Ser. No. i78,247 Claims priority, application Germany, Mar. 15, 1961, M 48,464 4 Claims. (Cl. 14S-6.15)

The present invention relates to an improved method for preparing metal surfaces to receive adherent films of water soluble base paint and a process of coating metal with smooth, adherent thermoset resin films, and more particularly relates to a method for rinsing a metal surface preliminarily treated in acidic solution to form thereon a protective integral chemical coating such as a phosphate, chromate, oxalate, or the like, and thereafter coating said surface with a water soluble thermosettable resin.

In the preparation of metal surfaces, particularly of ferrous surfaces for receiving finish coatings such as paint, enamel, varnish or the like, it is now conventional to subject that metal surface to preliminary cleaning, acid etching or to treatment with aqueous acidic solutions which form on the surface an integral chemically bound coating which serves to bond the finish coating securely to the metal surface. In this preliminary chemical treatment it has been conventional heretofore to complete the aqueous acidic solution treatment with a rinse which, in most cases, is an aqueous acidic solution containing the hexavalent chromium ion, phosphoric acid or mixtures thereof. In some processes, only a final water rinse is employed, but in either case the coating contains entrapped acid in variable quantity. Following the final rinse, the coating is ordinarily dried prior to the application of the finish coating of lacquer, varnish or paint and at least in those high quantity production lines which fabricate automobile bodies, refrigerator shells, stoves and the like, the final coating is applied by spraying. With the recent availability of water soluble paint systems it has become feasible to eliminate the drying step and to apply a water soluble paint on the chemically prepared metallic surface immediately following the final rinse in the chemical treatment. It has been found that in such processes that difficulties are encountered during operation including the formation of irregularities or roughness in the final cured film or the precipitation of conglomerates or sludge within the tank or receptacle which houses the water soluble paint.

It is the primary object of this invention to provide an improved process which enables the application of water soluble resins, paints, varnishes or the like to metal surfaces previously treated in acidic solutions without encountering the above types of difficulty.

ln accordance with this invention it has been found that this difficulty is overcome when the final stepI in the chemical treatment of the metal surface preparatory to its contact with the water soluble paint comprises rinsing that surface in an aqueous solution having a pH in the range of about 7 to about ll, and preferably rinsing that surface in an aqueous solution of an amine.

The process of this invention is set forth in iiowsheet form in the drawing. ln the drawing, the process consisting of the steps connected by solid lines is the simplest form of this invention, whereas, the steps connected by dotted lines represent alternative forms for use when desired.

Amines which have been found to be suitable for the purposes of this invention include ammonia, diethylamine, triethylamine, triethanolamine, morpholine, cyclohexylamine, dicyclohexylamine, aminopropanol, and

BMABS Patented ct. 27, 1964 ice aminoisobutanol. Such amine solutions may satisfactorily contain other anions, such as the phosphate anion, or amine phosphate esters of monoand di-phosphoric acids such as, for example, monomethyl amine phosphate, ethyl amine phosphate, isopropyl amine phosphate, monoethanol amine phosphate, diamyl amine phosphate, 2 amino-Z-methyl-l-propanol amine phosphate, cyclohexylamine phosphate, tertiary octyl amine phosphate, and ethylene diamine phosphate. The rinse may also include small quantities of alcohol or other compatible solvent and a small quantity of a non-ionic, cationic or anionic wetting agent. The concentration of the amine which is best for use in any particular case is dependent upon its inherent alkalinity and solubility in water and the optimum quantity for use is an amount which will produce the pH in the range of about 7-10 and preferably in the range of 8-10.

In the formation of chemically produced integrally bound coatings such as phosphate or oxalates which have substantial weight per unit of surface area and which are particulate in nature with space between the particles which serve to adsorb or trap liquid applied to such a coating, it is undesirable to employ the now conventional aqueous acidic rinses which are of value for the purpose of enhancing corrosion resistance and the quality of the coating as a base for paint, such as for example, a dilute aqueous hexavalent chromium containing solution or a phosphoric acid containing solution or tthe like. The entrapped acidic material which results from such a rinse, is particularly damaging to the quality of the Water soluble paint when the freshly rinsed chemically coated surface is thereafter dipped or immersed in the water soluble paint tank and such a procedure results in a precipitation of the components of the water soluble paint in the application tank and an ultimate destruction of the quality of the water based paint film. Moreover, water soluble paint, lacquer or varnishes sprayed or atomized on such an acidic surface coating produces a rough, non-uniform film which is apparent after heat curing of the resin.

It is undesirable to include in the aqueous alkaline rinse of this invention any material, inorganic or organic which after curing of the water soluble paint film remain in the film in a form which is soluble in Water since the leaching of such material from the final film reduces the corrosion resistance of the total film to humid conditions. For this reason it is advantageous to employ distilled water or water which has been deionized for the preparation of the rinse solutions to this invention.

All of the known types of water soluble resins have been found to be improved by the preliminary use of the rinse solutions of this invention including, for example, the water soluble phenolic resins, water soluble acrylic resins, water soluble melamine resins, water soluble alkyd phenolic resins, the water soluble alkyd melamine resins, the water soluble melamine modified acrylic resins, the water soluble fatty acid modified alkyd resins, etc. More specific examples of these types which have been found to be improved in use in a process involving phosphate coating, rinsing and water soluble paint coating include the melamine modified acrylic resins available under the trade name Melaqua 600 from American Cyanamid Company, the phenolic resin available under the trade name Resydrol P-41l, and the melamine resins available commercially under the trade name Resydroles M-470 and M-47l, which are available from Reichhold, Chemie A. G. Other specific resins of this type include the fatty acid free phthalate resin available under the trade name Alftalat 420 A, having a melting range of 55-60 C., an acid number of -200 and being water clear in appearance and available from Chemische Werke, Albert, and the fatty acid modified alkyd resin available under the trade name Alftalat liti. 329/4 and having an acid es number of 80-90, viscosity, at 50% concentration in toluol, of (170 centipoises and containing about 50% fatty acid modication, which is available from Chemische Werke, Albert.

The water soluble paint systems may include in addition to the resins of the type above specified conventional pigments, flow irnprovers, softening agents and solvents including alcohols, glycols, etc., in addition to water, and such other ingredients may be present in the proportions conventionally used in such paint systems.

The following examples are intended to illustrate in somewhat greater detail the process of this invention, although it is to be understood that the examples are for purposes of illustration only and do not set forth the definitive limits of the materials, proportions or operating conditions of this invention which have been hereinabove specified.

Example I An acidic zinc phosphate solution of the type conventionally employed to form a phosphate coating for use as a base for paint was compounded by admixing with water, zinc oxide, 75% phosphoric acid and nitric acid in quantities to produce a solution having an analysis of 0.14% Zn, 0.75% P04 and 1.8% NO3. The solution had a total acid of 10.1.

With the solution at 135 F., coatings were formed by spraying the solution on the surface of conventionally cleaned cold rolled steel panels, 4 x 6, in a contact time of about l minute. The average coating weight produced was about 265 milligrams per square foot. Some of the panels were then rinsed in a dilute aqueous hexavalent chromium-containing solution, formulated at 0.03% CrO3, removed and cold water rinsed. Certain of the phosphate coated panels were rinsed in an aqueous ammonium hydroxide solution having a pH of 8.5, and a plurality of each of these panels were spray-coated with a water soluble paint having the following composition:

150 parts by weight water soluble phenolic resin (Resydrol P-4l 1 610 parts water;

195 parts lithopone;

5 parts ferric oxide, and

parts butanol.

The panels, after spraying, were air dried and then subjected to a S-minute heat cure in an oven at 150 C. and removed. After cooling, the panels were inspected and each were found to be coated with a gray continuous film but it was apparent that the panels which were given the chromic acid rinse were rougher and contained interspersed globular areas whereas the panels which were rinsed in the ammonium hydroxide rinse prior to spraying were coated with a smooth continuous gray film.

Continuous processing of a plurality of panels, rinsed in the above identified chromic acid solution, by dipping those panels, immediately after the water rinse following the chromic acid rinse treatment, into the above specified water base paint composition resulted in a short time in the formation of precipitate in the water-based paint in its container, which could be visibly seen and which slowly settled toward the bottom of the container. A similar quantity of panels rinsed in the aqueous ammonium hydroxide solution processed through another similar size tank containing the identical water soluble paint showed no precipitation.

Other suitable paint systems which were found to work satisfactorily with the aqueous amine rinse treatment of this composition, following phosphate coatings, are given in the examples which follow:

Example II A water soluble paint was formulated to contain 150 parts by weight melamine resin (Resydrol M-471), 95 parts titanium dioxide, 5 parts zinc sulfide, 15 parts isopropanol and 440 parts water.

After this water soluble paint was admixed and allowed to stand for about 2 hours, application by spraying of it to zinc phosphate coated panels of the type specified in Ex ample I, subjected to the aqueous ammonium hydroxide rinse having a pH of 8.5, after curing for 5 minutes at C., were found to form smooth uniform white continuous paint films over the entire surfaces of the panels.

Example III An acrylic polymer emulsion was prepared using Rhoplex AC-200 acrylic resin. This material as used contains 46% solids, a pH of 9-l0, has a bulking value of 0.110 gallon per pound of emulsion and 0.0985 gallon per pound of solid polymer. A spray formulation containing this resin was prepared containing, in pounds per gallon of water, 7.2 pounds Rhoplex AC-200 (46% solids), .36 pound isophorone, 2 pounds rutile titanium dioxide, .02 pound morpholine, 0.01 pound tributyl phosphate, 0.02 pound dimethyl amino ethanol, and 0.8 pound water. The resultant dispersion had a pH of about 9.5 and was colored slightly blue by adding a trace of phthalocyanine blue.

Cold rolled steel panels, conventionally cleaned, were phosphate coated in the zinc phosphate solution specified above in Example I, water rinsed, and then rinsed in a dilute aqueous solution containing a mixture of ethylamine phosphate and ammonium hydroxide and having a pH of 8.0. After removal and drying, the panels were spray coated with the aqueous emulsion formulation above identified, and the baked at 350 for 30 minutes. An inspection of the coatings on the surfaces, after cooling, showed the surfaces to be coated with a smooth white continuous film free of undulations, conglomerates or other roughness.

As generally indicated above, the preliminary chemical treatment to which the metal surface is subjected may include aqueous acidic solutions which function as cleaners or aqueous acidic solutions which function to form an integrally bound chemically induced coating on the metal surface. The rinsing step of this invention is useful following a preliminary contact of the metal surface with an aqueous acidic solution of phosphoric, chromic or oxalic acid or from the preliminary treatment of the surface with an aqueous acidic solution of zinc phosphate, manganese phosphate, one or more alkaline earth metal phosphates, or an alkali metal phosphate, including ammonium phosphate, or Zinc or manganese phosphate modified with an alkaline earth metal phosphate or an alkali metal phosphate including ammonium phosphate.

The best thermosetting or curing temperature for use in curing the water soluble paint, lacquer or varnish materials of this invention will vary with the particular material selected but, in general, a temperature in the range of 200 C. to about 300 C. is satisfactory.

What is claimed is:

1. A process for preparing a metal surface for receiving a water soluble paint film which comprises the steps of (1) contacting said surface with an aqueous acidic solution of at least one phosphate selected from the group consisting of zinc phosphate, manganese phosphate, alkaline earth metal phosphates and the alkali metal phosphates, (2) thereafter rinsing the surface of the metal from step (l) in an aqueous acidic solution of an acid selected from the group consisting of phosphoric, chromic and oxalic acids, (3) thereafter rinsing the surface of the metal from step (2) in an aqueous rinse solution of an amine selected from the group consisting of ammonia, diethylamine, triethylamine, triethanolamine, morpholine, cyclohexylamine, dicyclohexylamine, aminopropanol and aminoisobutanol, said rinse solution having a pH in the range of 7 to about l1, and (4) while said rinsed surface is wet, applying `a water soluble paint to the surface of the metal from step (3) and curing said water soluble paint to thereby form a continuous film on said surface.

2. A process for preparing a metal surface for receiving a Water soluble paint ilm which comprises the steps of (1) contacting said surface with an aqueous -acidic solution of a material selected from the group consisting of phosphoric acid, chromic acid, oxalic acid, zinc phosphate, manganese phosphate, alkaline earth metal phosphates and alkali metal phosphates and mixtures of said phosphates and (2) thereafter rinsing the surface of the metal from step (1) in an aqueous rinse solution of an amine selected from the group consisting of ammonia, diethylamine, triethylamine, triethanolamine, morpholine, cyclohexylamine, dicyclohexylamine, aminopropanol and aminoisobutanol, said rinse solution having a pH in the range of 7 to about 11, and (3) While said rinsed surface is Wet, applying a Water soluble paint to said surface and curing said Water soluble paint to thereby form a continuous lm 15 on said surface.

3. A process in accordance with claim 2 wherein said amine solution includes at least one phosphate ester selectJ ed from the group consisting of monomethyl amine phos phate, ethyl amine phosphate, isopropyl amine phosphate, monoethanol amine phosphate, diamyl amine phosphate, 2 amino-Z-methyl-l-propanol amine phosphate, cyclohexyh amine phosphate, tertiary octyl amine phosphate land ethylene diamine phosphate.

4. A process in accordance with claim 2 wherein said aqueous acidic solution is zinc phosphate.

References Cited in the file of this patent UNITED STATES PATENTS 2,418,608 Thompson et al Apr. 8, 1947 2,768,103 Schuster et al Oct. 23, 1956 2,768,104 Schuster et al Oct. 23, 1956 

1. A PROCESS FOR PREPARING A METAL SURFACE FOR RECEIVING A WATER SOLUBLE PAINT FILM WHICH COMPRISES THE STEPS OF (1) CONTACTING SAID SURFACE WITH AN AQUEOUS ACIDIC SOLUTION OF AT LEAST ONE PHOSPHATE SELECTED FROM THE GROUP CONSISTING OF ZINC PHOSPHATE, MANGANESE PHOSPHATE, ALKALINE EARTH METAL PHOSPHATES AND THE ALKALI METAL PHOSPHATES, (2) THEREAFTER RINSING THE SURFACE OF THE METAL FROM STEP (1) IN AN AQUEOUS ACIDIC SOLUTION OF AN ACID SELECTED FROM THE GROUP CONSISTING OF PHOSPHORIC, CHROMIC AND OXALIC ACIDS, (3) THEREAFTER RINSING THE SURFACE OF THE METAL FROM STEP (2) IN AN AQUEOUS RINSE SOLUTION OF AN AMINE SELECTED FROM THE GROUP CONSISTING OF AMMONIA, DIETHYLAMINE, TRIETHYLAMINE, TRIETHANOLAMINE, MORPHOLINE, CYCLOHEXYLAMINE, DICYCLOHEXYLAMINE, AMINOPROPANOL AND AMINOISOBUTANOL, SAID RINSE SOLUTION HAVING A PH IN THE RANGE OF 7 TO ABOUT 11, AND (4) WHILE SAID RINSED SURFACE IS WET, APPLYING A WATER SOLUBLE PAINT TO THE SURFACE OF THE METAL 